The present invention relates to injection molding machines, and in particular, a sprue bar shutoff method used in a system that transports molten polymer from the molding machine to at least one stack mold by way of a sprue bar.
It is known to provide stack molds in injection molding machines in which there are at least two sets of molding chambers which divide along separate parting lines. One method of delivering molten polymer to all of the molding chambers in a stack mold is by a sprue bar. A sprue bar extends from a sprue of the molding machine, along the axis of the stack mold assembly, to a central manifold located between at least two mold plates in the stack. A plurality of nozzles in turn provide conduits for the molten polymer from the manifold to each of the molding cavities, with some of the nozzles extending forward in an axial direction to those mold cavities which are forward of the manifold, and the rest of the nozzles extending rearward in an opposite axial direction to those mold cavities which are rearward of the manifold. A representative sprue bar-equipped stack mold is illustrated in FIG. 2 of U.S. Pat. No. 7,427,197 to Ciccone et al.
After the molten polymer has been injected under pressure into the molding chambers, the system is depressurized, the cavities are cooled, the plates forming the molding chambers are separated, and the molded piece or pieces are ejected. During this process it is necessary to disconnect the sprue from the sprue bar. Upon disconnection, molten polymer or “drool” will often be produced from the orifice of the sprue bar, resulting in the loss of raw material and additional cleanup costs. If enough molten polymer drools from the sprue bar, it may solidify and prevent complete closure of the mold, causing damage to the equipment, expensive repairs, and lost production. At the very least, this drooling effect requires removal by a technician, increasing the time between cycles. Thus, it would be desirable to provide a sprue bar shutoff device that reduces the amount of drool or prevents it completely.